1. Field of the Invention
The invention is concerned with the field of combustion technology. It relates to a method for operating an annular combustion chamber which is equipped with premixing burners and which is used particularly for gas turbines.
2. Discussion of Background
In the light of the low pollutant emission values prescribed for reasons of environmental protection, premixing burners are increasingly used in the operation of gas turbines.
One of the disadvantages of premixing burners is that they are extinguished even when the air coefficients are very low. Depending on the temperature downstream of the compressor of the gas turbine group, this air coefficient .lambda. is around 2.
In order to achieve minimal NOx emissions in the case of lean premixing burners, it is customary, under full load, to operate all the burners under identical stoichiometric conditions near the lean extinction limit. Problems arise when the load falls below the full-load value, because, without corresponding countermeasures, the extinction limit is then exceeded, thus leading to the blowout of the premixing burners. Such known countermeasures for flame stabilization are, for example, a redistribution of fuel and/or the changeover to another flame mode and/or the use of the air bypass technique.
For the purpose of applying the "redistribution of fuel" principle, the fuel system is divided into branches which can be shut off or throttled individually. The air flow path remains unchanged with a decreasing load. When the total fuel quantity decreases, some of the valves are closed completely or partially, so that the open parts receive a larger percentage of fuel. This portion then results in the actual flame stabilization.
Another known possibility for maintaining combustion in the low load range is to change over to other flame modes which still guarantee stable combustion at a higher air/fuel ratio, for example diffusion gas stages. A mixed mode, in which the premixing flames are partially enriched, is also known. Complete burnout at air coefficients below 3.5 is thereby possible, that is to say the operating range of 2 to 3.5 can be covered. The higher NOx emissions are a disadvantage here.
Finally, the air bypass technique can also be employed to prevent the blowout of the premixing burners under low loads. For this purpose, as is known, variable orifices, through which the air can flow in in a regulated manner, are arranged in the combustion chamber downstream of the flame. The orifices are closed under full load and are opened with a decreasing load. The aim is to keep the adiabatic flame temperature and therefore also the distance from the extinction limit constant. In many instances, a similar possibility can be achieved by means of a throttle member arranged in the air supply to the burner.
The disadvantage of this prior art which has just been described is that these mechanisms necessitate a complicated fuel distribution and regulation system.
It is desirable, in principle, to operate all the burners in the premixing mode, even under low loads. This can be achieved by a reduction in the mass airflow through the machine (adjustable guide blade stages in the compressor). A known problem which occurs repeatedly is the rise of the gas turbine outflow temperature with a fall in load beyond the predetermined level which puts the lower operating point outside the range within which the premixing combustion principle can be operated.